Traditionally, an upside down apple was the secret symbol of homosexual sexual desire...of anal lust...'cause the bottome of an apple resembles the anus. And of course, in the bible tradition, the apple was the downfall of Adam...like boys are the downfall of men who fall in lust with them.

(I've got a feeling I've said this before but...) The fruit of the medlar tree is a more specific reminder of not the whole arse but only one part of it - the arsehole or anus. (You can see photos of the fruit on the internet.) The traditional English name for it, used by Chaucer and Shakespeare and until more puritanical times recently, was OPEN-ARSE, presumably referring to the delightful vision of an arsehole when the buttocks are pulled apart. In French, on the other hand, it was and is called CUL DE CHIEN (dog's arse) or CUL DE SINGE (monkey's arse) with no reference to an opening action. Perhaps because dogs' and monkeys' anuses are on permanent display.